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</head>
<body class="article">
<div id="header">
<h1>Git hash function transition</h1>
</div>
<div id="content">
<div class="sect1">
<h2 id="_objective">Objective</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Migrate Git from SHA-1 to a stronger hash function.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_background">Background</h2>
<div class="sectionbody">
<div class="paragraph">
<p>At its core, the Git version control system is a content addressable
filesystem. It uses the SHA-1 hash function to name content. For
example, files, directories, and revisions are referred to by hash
values unlike in other traditional version control systems where files
or versions are referred to via sequential numbers. The use of a hash
function to address its content delivers a few advantages:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Integrity checking is easy. Bit flips, for example, are easily
detected, as the hash of corrupted content does not match its name.</p>
</li>
<li>
<p>Lookup of objects is fast.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Using a cryptographically secure hash function brings additional
advantages:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Object names can be signed and third parties can trust the hash to
address the signed object and all objects it references.</p>
</li>
<li>
<p>Communication using Git protocol and out of band communication
methods have a short reliable string that can be used to reliably
address stored content.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Over time some flaws in SHA-1 have been discovered by security
researchers. On 23 February 2017 the SHAttered attack
(<a href="https://shattered.io" class="bare">https://shattered.io</a>) demonstrated a practical SHA-1 hash collision.</p>
</div>
<div class="paragraph">
<p>Git v2.13.0 and later subsequently moved to a hardened SHA-1
implementation by default, which isn&#8217;t vulnerable to the SHAttered
attack, but SHA-1 is still weak.</p>
</div>
<div class="paragraph">
<p>Thus it&#8217;s considered prudent to move past any variant of SHA-1
to a new hash. There&#8217;s no guarantee that future attacks on SHA-1 won&#8217;t
be published in the future, and those attacks may not have viable
mitigations.</p>
</div>
<div class="paragraph">
<p>If SHA-1 and its variants were to be truly broken, Git&#8217;s hash function
could not be considered cryptographically secure any more. This would
impact the communication of hash values because we could not trust
that a given hash value represented the known good version of content
that the speaker intended.</p>
</div>
<div class="paragraph">
<p>SHA-1 still possesses the other properties such as fast object lookup
and safe error checking, but other hash functions are equally suitable
that are believed to be cryptographically secure.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_choice_of_hash">Choice of Hash</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The hash to replace the hardened SHA-1 should be stronger than SHA-1
was: we would like it to be trustworthy and useful in practice for at
least 10 years.</p>
</div>
<div class="paragraph">
<p>Some other relevant properties:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>A 256-bit hash (long enough to match common security practice; not
excessively long to hurt performance and disk usage).</p>
</li>
<li>
<p>High quality implementations should be widely available (e.g., in
OpenSSL and Apple CommonCrypto).</p>
</li>
<li>
<p>The hash function&#8217;s properties should match Git&#8217;s needs (e.g. Git
requires collision and 2nd preimage resistance and does not require
length extension resistance).</p>
</li>
<li>
<p>As a tiebreaker, the hash should be fast to compute (fortunately
many contenders are faster than SHA-1).</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>There were several contenders for a successor hash to SHA-1, including
SHA-256, SHA-512/256, SHA-256x16, K12, and BLAKE2bp-256.</p>
</div>
<div class="paragraph">
<p>In late 2018 the project picked SHA-256 as its successor hash.</p>
</div>
<div class="paragraph">
<p>See 0ed8d8da374 (doc hash-function-transition: pick SHA-256 as
NewHash, 2018-08-04) and numerous mailing list threads at the time,
particularly the one starting at
<a href="https://lore.kernel.org/git/20180609224913.GC38834@genre.crustytoothpaste.net/" class="bare">https://lore.kernel.org/git/20180609224913.GC38834@genre.crustytoothpaste.net/</a>
for more information.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_goals">Goals</h2>
<div class="sectionbody">
<div class="olist arabic">
<ol class="arabic">
<li>
<p>The transition to SHA-256 can be done one local repository at a time.</p>
<div class="olist loweralpha">
<ol class="loweralpha">
<li>
<p>Requiring no action by any other party.</p>
</li>
<li>
<p>A SHA-256 repository can communicate with SHA-1 Git servers
(push/fetch).</p>
</li>
<li>
<p>Users can use SHA-1 and SHA-256 identifiers for objects
interchangeably (see "Object names on the command line", below).</p>
</li>
<li>
<p>New signed objects make use of a stronger hash function than
SHA-1 for their security guarantees.</p>
</li>
</ol>
</div>
</li>
<li>
<p>Allow a complete transition away from SHA-1.</p>
<div class="olist loweralpha">
<ol class="loweralpha">
<li>
<p>Local metadata for SHA-1 compatibility can be removed from a
repository if compatibility with SHA-1 is no longer needed.</p>
</li>
</ol>
</div>
</li>
<li>
<p>Maintainability throughout the process.</p>
<div class="olist loweralpha">
<ol class="loweralpha">
<li>
<p>The object format is kept simple and consistent.</p>
</li>
<li>
<p>Creation of a generalized repository conversion tool.</p>
</li>
</ol>
</div>
</li>
</ol>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_non_goals">Non-Goals</h2>
<div class="sectionbody">
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Add SHA-256 support to Git protocol. This is valuable and the
logical next step but it is out of scope for this initial design.</p>
</li>
<li>
<p>Transparently improving the security of existing SHA-1 signed
objects.</p>
</li>
<li>
<p>Intermixing objects using multiple hash functions in a single
repository.</p>
</li>
<li>
<p>Taking the opportunity to fix other bugs in Git&#8217;s formats and
protocols.</p>
</li>
<li>
<p>Shallow clones and fetches into a SHA-256 repository. (This will
change when we add SHA-256 support to Git protocol.)</p>
</li>
<li>
<p>Skip fetching some submodules of a project into a SHA-256
repository. (This also depends on SHA-256 support in Git
protocol.)</p>
</li>
</ol>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_overview">Overview</h2>
<div class="sectionbody">
<div class="paragraph">
<p>We introduce a new repository format extension. Repositories with this
extension enabled use SHA-256 instead of SHA-1 to name their objects.
This affects both object names and object content&#8201;&#8212;&#8201;both the names
of objects and all references to other objects within an object are
switched to the new hash function.</p>
</div>
<div class="paragraph">
<p>SHA-256 repositories cannot be read by older versions of Git.</p>
</div>
<div class="paragraph">
<p>Alongside the packfile, a SHA-256 repository stores a bidirectional
mapping between SHA-256 and SHA-1 object names. The mapping is generated
locally and can be verified using "git fsck". Object lookups use this
mapping to allow naming objects using either their SHA-1 and SHA-256 names
interchangeably.</p>
</div>
<div class="paragraph">
<p>"git cat-file" and "git hash-object" gain options to display an object
in its SHA-1 form and write an object given its SHA-1 form. This
requires all objects referenced by that object to be present in the
object database so that they can be named using the appropriate name
(using the bidirectional hash mapping).</p>
</div>
<div class="paragraph">
<p>Fetches from a SHA-1 based server convert the fetched objects into
SHA-256 form and record the mapping in the bidirectional mapping table
(see below for details). Pushes to a SHA-1 based server convert the
objects being pushed into SHA-1 form so the server does not have to be
aware of the hash function the client is using.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_detailed_design">Detailed Design</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_repository_format_extension">Repository format extension</h3>
<div class="paragraph">
<p>A SHA-256 repository uses repository format version <code>1</code> (see
Documentation/technical/repository-version.txt) with extensions
<code>objectFormat</code> and <code>compatObjectFormat</code>:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>[core]
	repositoryFormatVersion = 1
[extensions]
	objectFormat = sha256
	compatObjectFormat = sha1</pre>
</div>
</div>
<div class="paragraph">
<p>The combination of setting <code>core.repositoryFormatVersion=1</code> and
populating <code>extensions.*</code> ensures that all versions of Git later than
<code>v0.99.9l</code> will die instead of trying to operate on the SHA-256
repository, instead producing an error message.</p>
</div>
<div class="literalblock">
<div class="content">
<pre># Between v0.99.9l and v2.7.0
$ git status
fatal: Expected git repo version &lt;= 0, found 1
# After v2.7.0
$ git status
fatal: unknown repository extensions found:
	objectformat
	compatobjectformat</pre>
</div>
</div>
<div class="paragraph">
<p>See the "Transition plan" section below for more details on these
repository extensions.</p>
</div>
</div>
<div class="sect2">
<h3 id="_object_names">Object names</h3>
<div class="paragraph">
<p>Objects can be named by their 40 hexadecimal digit SHA-1 name or 64
hexadecimal digit SHA-256 name, plus names derived from those (see
gitrevisions(7)).</p>
</div>
<div class="paragraph">
<p>The SHA-1 name of an object is the SHA-1 of the concatenation of its
type, length, a nul byte, and the object&#8217;s SHA-1 content. This is the
traditional &lt;sha1&gt; used in Git to name objects.</p>
</div>
<div class="paragraph">
<p>The SHA-256 name of an object is the SHA-256 of the concatenation of its
type, length, a nul byte, and the object&#8217;s SHA-256 content.</p>
</div>
</div>
<div class="sect2">
<h3 id="_object_format">Object format</h3>
<div class="paragraph">
<p>The content as a byte sequence of a tag, commit, or tree object named
by SHA-1 and SHA-256 differ because an object named by SHA-256 name refers to
other objects by their SHA-256 names and an object named by SHA-1 name
refers to other objects by their SHA-1 names.</p>
</div>
<div class="paragraph">
<p>The SHA-256 content of an object is the same as its SHA-1 content, except
that objects referenced by the object are named using their SHA-256 names
instead of SHA-1 names. Because a blob object does not refer to any
other object, its SHA-1 content and SHA-256 content are the same.</p>
</div>
<div class="paragraph">
<p>The format allows round-trip conversion between SHA-256 content and
SHA-1 content.</p>
</div>
</div>
<div class="sect2">
<h3 id="_object_storage">Object storage</h3>
<div class="paragraph">
<p>Loose objects use zlib compression and packed objects use the packed
format described in <a href="../gitformat-pack.html">gitformat-pack(5)</a>, just like
today. The content that is compressed and stored uses SHA-256 content
instead of SHA-1 content.</p>
</div>
</div>
<div class="sect2">
<h3 id="_pack_index">Pack index</h3>
<div class="paragraph">
<p>Pack index (.idx) files use a new v3 format that supports multiple
hash functions. They have the following format (all integers are in
network byte order):</p>
</div>
<div class="ulist">
<ul>
<li>
<p>A header appears at the beginning and consists of the following:</p>
<div class="ulist">
<ul>
<li>
<p>The 4-byte pack index signature: <em>\377t0c</em></p>
</li>
<li>
<p>4-byte version number: 3</p>
</li>
<li>
<p>4-byte length of the header section, including the signature and
version number</p>
</li>
<li>
<p>4-byte number of objects contained in the pack</p>
</li>
<li>
<p>4-byte number of object formats in this pack index: 2</p>
</li>
<li>
<p>For each object format:</p>
<div class="ulist">
<ul>
<li>
<p>4-byte format identifier (e.g., <em>sha1</em> for SHA-1)</p>
</li>
<li>
<p>4-byte length in bytes of shortened object names. This is the
shortest possible length needed to make names in the shortened
object name table unambiguous.</p>
</li>
<li>
<p>4-byte integer, recording where tables relating to this format
are stored in this index file, as an offset from the beginning.</p>
</li>
</ul>
</div>
</li>
<li>
<p>4-byte offset to the trailer from the beginning of this file.</p>
</li>
<li>
<p>Zero or more additional key/value pairs (4-byte key, 4-byte
value). Only one key is supported: <em>PSRC</em>. See the "Loose objects
and unreachable objects" section for supported values and how this
is used.  All other keys are reserved. Readers must ignore
unrecognized keys.</p>
</li>
</ul>
</div>
</li>
<li>
<p>Zero or more NUL bytes. This can optionally be used to improve the
alignment of the full object name table below.</p>
</li>
<li>
<p>Tables for the first object format:</p>
<div class="ulist">
<ul>
<li>
<p>A sorted table of shortened object names.  These are prefixes of
the names of all objects in this pack file, packed together
without offset values to reduce the cache footprint of the binary
search for a specific object name.</p>
</li>
<li>
<p>A table of full object names in pack order. This allows resolving
a reference to "the nth object in the pack file" (from a
reachability bitmap or from the next table of another object
format) to its object name.</p>
</li>
<li>
<p>A table of 4-byte values mapping object name order to pack order.
For an object in the table of sorted shortened object names, the
value at the corresponding index in this table is the index in the
previous table for that same object.
This can be used to look up the object in reachability bitmaps or
to look up its name in another object format.</p>
</li>
<li>
<p>A table of 4-byte CRC32 values of the packed object data, in the
order that the objects appear in the pack file. This is to allow
compressed data to be copied directly from pack to pack during
repacking without undetected data corruption.</p>
</li>
<li>
<p>A table of 4-byte offset values. For an object in the table of
sorted shortened object names, the value at the corresponding
index in this table indicates where that object can be found in
the pack file. These are usually 31-bit pack file offsets, but
large offsets are encoded as an index into the next table with the
most significant bit set.</p>
</li>
<li>
<p>A table of 8-byte offset entries (empty for pack files less than
2 GiB). Pack files are organized with heavily used objects toward
the front, so most object references should not need to refer to
this table.</p>
</li>
</ul>
</div>
</li>
<li>
<p>Zero or more NUL bytes.</p>
</li>
<li>
<p>Tables for the second object format, with the same layout as above,
up to and not including the table of CRC32 values.</p>
</li>
<li>
<p>Zero or more NUL bytes.</p>
</li>
<li>
<p>The trailer consists of the following:</p>
<div class="ulist">
<ul>
<li>
<p>A copy of the 20-byte SHA-256 checksum at the end of the
corresponding packfile.</p>
</li>
<li>
<p>20-byte SHA-256 checksum of all of the above.</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_loose_object_index">Loose object index</h3>
<div class="paragraph">
<p>A new file $GIT_OBJECT_DIR/loose-object-idx contains information about
all loose objects. Its format is</p>
</div>
<div class="literalblock">
<div class="content">
<pre># loose-object-idx
(sha256-name SP sha1-name LF)*</pre>
</div>
</div>
<div class="paragraph">
<p>where the object names are in hexadecimal format. The file is not
sorted.</p>
</div>
<div class="paragraph">
<p>The loose object index is protected against concurrent writes by a
lock file $GIT_OBJECT_DIR/loose-object-idx.lock. To add a new loose
object:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Write the loose object to a temporary file, like today.</p>
</li>
<li>
<p>Open loose-object-idx.lock with O_CREAT | O_EXCL to acquire the lock.</p>
</li>
<li>
<p>Rename the loose object into place.</p>
</li>
<li>
<p>Open loose-object-idx with O_APPEND and write the new object</p>
</li>
<li>
<p>Unlink loose-object-idx.lock to release the lock.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>To remove entries (e.g. in "git pack-refs" or "git-prune"):</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Open loose-object-idx.lock with O_CREAT | O_EXCL to acquire the
lock.</p>
</li>
<li>
<p>Write the new content to loose-object-idx.lock.</p>
</li>
<li>
<p>Unlink any loose objects being removed.</p>
</li>
<li>
<p>Rename to replace loose-object-idx, releasing the lock.</p>
</li>
</ol>
</div>
</div>
<div class="sect2">
<h3 id="_translation_table">Translation table</h3>
<div class="paragraph">
<p>The index files support a bidirectional mapping between SHA-1 names
and SHA-256 names. The lookup proceeds similarly to ordinary object
lookups. For example, to convert a SHA-1 name to a SHA-256 name:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Look for the object in idx files. If a match is present in the
idx&#8217;s sorted list of truncated SHA-1 names, then:</p>
<div class="olist loweralpha">
<ol class="loweralpha">
<li>
<p>Read the corresponding entry in the SHA-1 name order to pack
name order mapping.</p>
</li>
<li>
<p>Read the corresponding entry in the full SHA-1 name table to
verify we found the right object. If it is, then</p>
</li>
<li>
<p>Read the corresponding entry in the full SHA-256 name table.
That is the object&#8217;s SHA-256 name.</p>
</li>
</ol>
</div>
</li>
<li>
<p>Check for a loose object. Read lines from loose-object-idx until
we find a match.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>Step (1) takes the same amount of time as an ordinary object lookup:
O(number of packs * log(objects per pack)). Step (2) takes O(number of
loose objects) time. To maintain good performance it will be necessary
to keep the number of loose objects low. See the "Loose objects and
unreachable objects" section below for more details.</p>
</div>
<div class="paragraph">
<p>Since all operations that make new objects (e.g., "git commit") add
the new objects to the corresponding index, this mapping is possible
for all objects in the object store.</p>
</div>
</div>
<div class="sect2">
<h3 id="_reading_an_objects_sha_1_content">Reading an object&#8217;s SHA-1 content</h3>
<div class="paragraph">
<p>The SHA-1 content of an object can be read by converting all SHA-256 names
of its SHA-256 content references to SHA-1 names using the translation table.</p>
</div>
</div>
<div class="sect2">
<h3 id="_fetch">Fetch</h3>
<div class="paragraph">
<p>Fetching from a SHA-1 based server requires translating between SHA-1
and SHA-256 based representations on the fly.</p>
</div>
<div class="paragraph">
<p>SHA-1s named in the ref advertisement that are present on the client
can be translated to SHA-256 and looked up as local objects using the
translation table.</p>
</div>
<div class="paragraph">
<p>Negotiation proceeds as today. Any "have"s generated locally are
converted to SHA-1 before being sent to the server, and SHA-1s
mentioned by the server are converted to SHA-256 when looking them up
locally.</p>
</div>
<div class="paragraph">
<p>After negotiation, the server sends a packfile containing the
requested objects. We convert the packfile to SHA-256 format using
the following steps:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>index-pack: inflate each object in the packfile and compute its
SHA-1. Objects can contain deltas in OBJ_REF_DELTA format against
objects the client has locally. These objects can be looked up
using the translation table and their SHA-1 content read as
described above to resolve the deltas.</p>
</li>
<li>
<p>topological sort: starting at the "want"s from the negotiation
phase, walk through objects in the pack and emit a list of them,
excluding blobs, in reverse topologically sorted order, with each
object coming later in the list than all objects it references.
(This list only contains objects reachable from the "wants". If the
pack from the server contained additional extraneous objects, then
they will be discarded.)</p>
</li>
<li>
<p>convert to SHA-256: open a new SHA-256 packfile. Read the topologically
sorted list just generated. For each object, inflate its
SHA-1 content, convert to SHA-256 content, and write it to the SHA-256
pack. Record the new SHA-1&#8592;&#8594;SHA-256 mapping entry for use in the idx.</p>
</li>
<li>
<p>sort: reorder entries in the new pack to match the order of objects
in the pack the server generated and include blobs. Write a SHA-256 idx
file</p>
</li>
<li>
<p>clean up: remove the SHA-1 based pack file, index, and
topologically sorted list obtained from the server in steps 1
and 2.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>Step 3 requires every object referenced by the new object to be in the
translation table. This is why the topological sort step is necessary.</p>
</div>
<div class="paragraph">
<p>As an optimization, step 1 could write a file describing what non-blob
objects each object it has inflated from the packfile references. This
makes the topological sort in step 2 possible without inflating the
objects in the packfile for a second time. The objects need to be
inflated again in step 3, for a total of two inflations.</p>
</div>
<div class="paragraph">
<p>Step 4 is probably necessary for good read-time performance. "git
pack-objects" on the server optimizes the pack file for good data
locality (see Documentation/technical/pack-heuristics.txt).</p>
</div>
<div class="paragraph">
<p>Details of this process are likely to change. It will take some
experimenting to get this to perform well.</p>
</div>
</div>
<div class="sect2">
<h3 id="_push">Push</h3>
<div class="paragraph">
<p>Push is simpler than fetch because the objects referenced by the
pushed objects are already in the translation table. The SHA-1 content
of each object being pushed can be read as described in the "Reading
an object&#8217;s SHA-1 content" section to generate the pack written by git
send-pack.</p>
</div>
</div>
<div class="sect2">
<h3 id="_signed_commits">Signed Commits</h3>
<div class="paragraph">
<p>We add a new field "gpgsig-sha256" to the commit object format to allow
signing commits without relying on SHA-1. It is similar to the
existing "gpgsig" field. Its signed payload is the SHA-256 content of the
commit object with any "gpgsig" and "gpgsig-sha256" fields removed.</p>
</div>
<div class="paragraph">
<p>This means commits can be signed</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>using SHA-1 only, as in existing signed commit objects</p>
</li>
<li>
<p>using both SHA-1 and SHA-256, by using both gpgsig-sha256 and gpgsig
fields.</p>
</li>
<li>
<p>using only SHA-256, by only using the gpgsig-sha256 field.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>Old versions of "git verify-commit" can verify the gpgsig signature in
cases (1) and (2) without modifications and view case (3) as an
ordinary unsigned commit.</p>
</div>
</div>
<div class="sect2">
<h3 id="_signed_tags">Signed Tags</h3>
<div class="paragraph">
<p>We add a new field "gpgsig-sha256" to the tag object format to allow
signing tags without relying on SHA-1. Its signed payload is the
SHA-256 content of the tag with its gpgsig-sha256 field and "-----BEGIN PGP
SIGNATURE-----" delimited in-body signature removed.</p>
</div>
<div class="paragraph">
<p>This means tags can be signed</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>using SHA-1 only, as in existing signed tag objects</p>
</li>
<li>
<p>using both SHA-1 and SHA-256, by using gpgsig-sha256 and an in-body
signature.</p>
</li>
<li>
<p>using only SHA-256, by only using the gpgsig-sha256 field.</p>
</li>
</ol>
</div>
</div>
<div class="sect2">
<h3 id="_mergetag_embedding">Mergetag embedding</h3>
<div class="paragraph">
<p>The mergetag field in the SHA-1 content of a commit contains the
SHA-1 content of a tag that was merged by that commit.</p>
</div>
<div class="paragraph">
<p>The mergetag field in the SHA-256 content of the same commit contains the
SHA-256 content of the same tag.</p>
</div>
</div>
<div class="sect2">
<h3 id="_submodules">Submodules</h3>
<div class="paragraph">
<p>To convert recorded submodule pointers, you need to have the converted
submodule repository in place. The translation table of the submodule
can be used to look up the new hash.</p>
</div>
</div>
<div class="sect2">
<h3 id="_loose_objects_and_unreachable_objects">Loose objects and unreachable objects</h3>
<div class="paragraph">
<p>Fast lookups in the loose-object-idx require that the number of loose
objects not grow too high.</p>
</div>
<div class="paragraph">
<p>"git gc --auto" currently waits for there to be 6700 loose objects
present before consolidating them into a packfile. We will need to
measure to find a more appropriate threshold for it to use.</p>
</div>
<div class="paragraph">
<p>"git gc --auto" currently waits for there to be 50 packs present
before combining packfiles. Packing loose objects more aggressively
may cause the number of pack files to grow too quickly. This can be
mitigated by using a strategy similar to Martin Fick&#8217;s exponential
rolling garbage collection script:
<a href="https://gerrit-review.googlesource.com/c/gerrit/+/35215" class="bare">https://gerrit-review.googlesource.com/c/gerrit/+/35215</a></p>
</div>
<div class="paragraph">
<p>"git gc" currently expels any unreachable objects it encounters in
pack files to loose objects in an attempt to prevent a race when
pruning them (in case another process is simultaneously writing a new
object that refers to the about-to-be-deleted object). This leads to
an explosion in the number of loose objects present and disk space
usage due to the objects in delta form being replaced with independent
loose objects.  Worse, the race is still present for loose objects.</p>
</div>
<div class="paragraph">
<p>Instead, "git gc" will need to move unreachable objects to a new
packfile marked as UNREACHABLE_GARBAGE (using the PSRC field; see
below). To avoid the race when writing new objects referring to an
about-to-be-deleted object, code paths that write new objects will
need to copy any objects from UNREACHABLE_GARBAGE packs that they
refer to new, non-UNREACHABLE_GARBAGE packs (or loose objects).
UNREACHABLE_GARBAGE are then safe to delete if their creation time (as
indicated by the file&#8217;s mtime) is long enough ago.</p>
</div>
<div class="paragraph">
<p>To avoid a proliferation of UNREACHABLE_GARBAGE packs, they can be
combined under certain circumstances. If "gc.garbageTtl" is set to
greater than one day, then packs created within a single calendar day,
UTC, can be coalesced together. The resulting packfile would have an
mtime before midnight on that day, so this makes the effective maximum
ttl the garbageTtl + 1 day. If "gc.garbageTtl" is less than one day,
then we divide the calendar day into intervals one-third of that ttl
in duration. Packs created within the same interval can be coalesced
together. The resulting packfile would have an mtime before the end of
the interval, so this makes the effective maximum ttl equal to the
garbageTtl * 4/3.</p>
</div>
<div class="paragraph">
<p>This rule comes from Thirumala Reddy Mutchukota&#8217;s JGit change
<a href="https://git.eclipse.org/r/90465" class="bare">https://git.eclipse.org/r/90465</a>.</p>
</div>
<div class="paragraph">
<p>The UNREACHABLE_GARBAGE setting goes in the PSRC field of the pack
index. More generally, that field indicates where a pack came from:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>1 (PACK_SOURCE_RECEIVE) for a pack received over the network</p>
</li>
<li>
<p>2 (PACK_SOURCE_AUTO) for a pack created by a lightweight
"gc --auto" operation</p>
</li>
<li>
<p>3 (PACK_SOURCE_GC) for a pack created by a full gc</p>
</li>
<li>
<p>4 (PACK_SOURCE_UNREACHABLE_GARBAGE) for potential garbage
discovered by gc</p>
</li>
<li>
<p>5 (PACK_SOURCE_INSERT) for locally created objects that were
written directly to a pack file, e.g. from "git add ."</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>This information can be useful for debugging and for "gc --auto" to
make appropriate choices about which packs to coalesce.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_caveats">Caveats</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_invalid_objects">Invalid objects</h3>
<div class="paragraph">
<p>The conversion from SHA-1 content to SHA-256 content retains any
brokenness in the original object (e.g., tree entry modes encoded with
leading 0, tree objects whose paths are not sorted correctly, and
commit objects without an author or committer). This is a deliberate
feature of the design to allow the conversion to round-trip.</p>
</div>
<div class="paragraph">
<p>More profoundly broken objects (e.g., a commit with a truncated "tree"
header line) cannot be converted but were not usable by current Git
anyway.</p>
</div>
</div>
<div class="sect2">
<h3 id="_shallow_clone_and_submodules">Shallow clone and submodules</h3>
<div class="paragraph">
<p>Because it requires all referenced objects to be available in the
locally generated translation table, this design does not support
shallow clone or unfetched submodules. Protocol improvements might
allow lifting this restriction.</p>
</div>
</div>
<div class="sect2">
<h3 id="_alternates">Alternates</h3>
<div class="paragraph">
<p>For the same reason, a SHA-256 repository cannot borrow objects from a
SHA-1 repository using objects/info/alternates or
$GIT_ALTERNATE_OBJECT_REPOSITORIES.</p>
</div>
</div>
<div class="sect2">
<h3 id="_git_notes">git notes</h3>
<div class="paragraph">
<p>The "git notes" tool annotates objects using their SHA-1 name as key.
This design does not describe a way to migrate notes trees to use
SHA-256 names. That migration is expected to happen separately (for
example using a file at the root of the notes tree to describe which
hash it uses).</p>
</div>
</div>
<div class="sect2">
<h3 id="_server_side_cost">Server-side cost</h3>
<div class="paragraph">
<p>Until Git protocol gains SHA-256 support, using SHA-256 based storage
on public-facing Git servers is strongly discouraged. Once Git
protocol gains SHA-256 support, SHA-256 based servers are likely not
to support SHA-1 compatibility, to avoid what may be a very expensive
hash re-encode during clone and to encourage peers to modernize.</p>
</div>
<div class="paragraph">
<p>The design described here allows fetches by SHA-1 clients of a
personal SHA-256 repository because it&#8217;s not much more difficult than
allowing pushes from that repository. This support needs to be guarded
by a configuration option --- servers like git.kernel.org that serve a
large number of clients would not be expected to bear that cost.</p>
</div>
</div>
<div class="sect2">
<h3 id="_meaning_of_signatures">Meaning of signatures</h3>
<div class="paragraph">
<p>The signed payload for signed commits and tags does not explicitly
name the hash used to identify objects. If some day Git adopts a new
hash function with the same length as the current SHA-1 (40
hexadecimal digit) or SHA-256 (64 hexadecimal digit) objects then the
intent behind the PGP signed payload in an object signature is
unclear:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>object e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7
type commit
tag v2.12.0
tagger Junio C Hamano &lt;gitster@pobox.com&gt; 1487962205 -0800</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>Git 2.12</pre>
</div>
</div>
<div class="paragraph">
<p>Does this mean Git v2.12.0 is the commit with SHA-1 name
e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7 or the commit with
new-40-digit-hash-name e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7?</p>
</div>
<div class="paragraph">
<p>Fortunately SHA-256 and SHA-1 have different lengths. If Git starts
using another hash with the same length to name objects, then it will
need to change the format of signed payloads using that hash to
address this issue.</p>
</div>
</div>
<div class="sect2">
<h3 id="_object_names_on_the_command_line">Object names on the command line</h3>
<div class="paragraph">
<p>To support the transition (see Transition plan below), this design
supports four different modes of operation:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>("dark launch") Treat object names input by the user as SHA-1 and
convert any object names written to output to SHA-1, but store
objects using SHA-256.  This allows users to test the code with no
visible behavior change except for performance.  This allows
running even tests that assume the SHA-1 hash function, to
sanity-check the behavior of the new mode.</p>
</li>
<li>
<p>("early transition") Allow both SHA-1 and SHA-256 object names in
input. Any object names written to output use SHA-1. This allows
users to continue to make use of SHA-1 to communicate with peers
(e.g. by email) that have not migrated yet and prepares for mode 3.</p>
</li>
<li>
<p>("late transition") Allow both SHA-1 and SHA-256 object names in
input. Any object names written to output use SHA-256. In this
mode, users are using a more secure object naming method by
default.  The disruption is minimal as long as most of their peers
are in mode 2 or mode 3.</p>
</li>
<li>
<p>("post-transition") Treat object names input by the user as
SHA-256 and write output using SHA-256. This is safer than mode 3
because there is less risk that input is incorrectly interpreted
using the wrong hash function.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>The mode is specified in configuration.</p>
</div>
<div class="paragraph">
<p>The user can also explicitly specify which format to use for a
particular revision specifier and for output, overriding the mode. For
example:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>git --output-format=sha1 log abac87a^{sha1}..f787cac^{sha256}</pre>
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_transition_plan">Transition plan</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Some initial steps can be implemented independently of one another:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>adding a hash function API (vtable)</p>
</li>
<li>
<p>teaching fsck to tolerate the gpgsig-sha256 field</p>
</li>
<li>
<p>excluding gpgsig-* from the fields copied by "git commit --amend"</p>
</li>
<li>
<p>annotating tests that depend on SHA-1 values with a SHA1 test
prerequisite</p>
</li>
<li>
<p>using "struct object_id", GIT_MAX_RAWSZ, and GIT_MAX_HEXSZ
consistently instead of "unsigned char *" and the hardcoded
constants 20 and 40.</p>
</li>
<li>
<p>introducing index v3</p>
</li>
<li>
<p>adding support for the PSRC field and safer object pruning</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>The first user-visible change is the introduction of the objectFormat
extension (without compatObjectFormat). This requires:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>teaching fsck about this mode of operation</p>
</li>
<li>
<p>using the hash function API (vtable) when computing object names</p>
</li>
<li>
<p>signing objects and verifying signatures</p>
</li>
<li>
<p>rejecting attempts to fetch from or push to an incompatible
repository</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Next comes introduction of compatObjectFormat:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>implementing the loose-object-idx</p>
</li>
<li>
<p>translating object names between object formats</p>
</li>
<li>
<p>translating object content between object formats</p>
</li>
<li>
<p>generating and verifying signatures in the compat format</p>
</li>
<li>
<p>adding appropriate index entries when adding a new object to the
object store</p>
</li>
<li>
<p>--output-format option</p>
</li>
<li>
<p>^{sha1} and ^{sha256} revision notation</p>
</li>
<li>
<p>configuration to specify default input and output format (see
"Object names on the command line" above)</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>The next step is supporting fetches and pushes to SHA-1 repositories:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>allow pushes to a repository using the compat format</p>
</li>
<li>
<p>generate a topologically sorted list of the SHA-1 names of fetched
objects</p>
</li>
<li>
<p>convert the fetched packfile to SHA-256 format and generate an idx
file</p>
</li>
<li>
<p>re-sort to match the order of objects in the fetched packfile</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>The infrastructure supporting fetch also allows converting an existing
repository. In converted repositories and new clones, end users can
gain support for the new hash function without any visible change in
behavior (see "dark launch" in the "Object names on the command line"
section). In particular this allows users to verify SHA-256 signatures
on objects in the repository, and it should ensure the transition code
is stable in production in preparation for using it more widely.</p>
</div>
<div class="paragraph">
<p>Over time projects would encourage their users to adopt the "early
transition" and then "late transition" modes to take advantage of the
new, more futureproof SHA-256 object names.</p>
</div>
<div class="paragraph">
<p>When objectFormat and compatObjectFormat are both set, commands
generating signatures would generate both SHA-1 and SHA-256 signatures
by default to support both new and old users.</p>
</div>
<div class="paragraph">
<p>In projects using SHA-256 heavily, users could be encouraged to adopt
the "post-transition" mode to avoid accidentally making implicit use
of SHA-1 object names.</p>
</div>
<div class="paragraph">
<p>Once a critical mass of users have upgraded to a version of Git that
can verify SHA-256 signatures and have converted their existing
repositories to support verifying them, we can add support for a
setting to generate only SHA-256 signatures. This is expected to be at
least a year later.</p>
</div>
<div class="paragraph">
<p>That is also a good moment to advertise the ability to convert
repositories to use SHA-256 only, stripping out all SHA-1 related
metadata. This improves performance by eliminating translation
overhead and security by avoiding the possibility of accidentally
relying on the safety of SHA-1.</p>
</div>
<div class="paragraph">
<p>Updating Git&#8217;s protocols to allow a server to specify which hash
functions it supports is also an important part of this transition. It
is not discussed in detail in this document but this transition plan
assumes it happens. :)</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_alternatives_considered">Alternatives considered</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_upgrading_everyone_working_on_a_particular_project_on_a_flag_day">Upgrading everyone working on a particular project on a flag day</h3>
<div class="paragraph">
<p>Projects like the Linux kernel are large and complex enough that
flipping the switch for all projects based on the repository at once
is infeasible.</p>
</div>
<div class="paragraph">
<p>Not only would all developers and server operators supporting
developers have to switch on the same flag day, but supporting tooling
(continuous integration, code review, bug trackers, etc) would have to
be adapted as well. This also makes it difficult to get early feedback
from some project participants testing before it is time for mass
adoption.</p>
</div>
</div>
<div class="sect2">
<h3 id="_using_hash_functions_in_parallel">Using hash functions in parallel</h3>
<div class="paragraph">
<p>(e.g. <a href="https://lore.kernel.org/git/22708.8913.864049.452252@chiark.greenend.org.uk/" class="bare">https://lore.kernel.org/git/22708.8913.864049.452252@chiark.greenend.org.uk/</a> )
Objects newly created would be addressed by the new hash, but inside
such an object (e.g. commit) it is still possible to address objects
using the old hash function.</p>
</div>
<div class="ulist">
<ul>
<li>
<p>You cannot trust its history (needed for bisectability) in the
future without further work</p>
</li>
<li>
<p>Maintenance burden as the number of supported hash functions grows
(they will never go away, so they accumulate). In this proposal, by
comparison, converted objects lose all references to SHA-1.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_signed_objects_with_multiple_hashes">Signed objects with multiple hashes</h3>
<div class="paragraph">
<p>Instead of introducing the gpgsig-sha256 field in commit and tag objects
for SHA-256 content based signatures, an earlier version of this design
added "hash sha256 &lt;SHA-256 name&gt;" fields to strengthen the existing
SHA-1 content based signatures.</p>
</div>
<div class="paragraph">
<p>In other words, a single signature was used to attest to the object
content using both hash functions. This had some advantages:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Using one signature instead of two speeds up the signing process.</p>
</li>
<li>
<p>Having one signed payload with both hashes allows the signer to
attest to the SHA-1 name and SHA-256 name referring to the same object.</p>
</li>
<li>
<p>All users consume the same signature. Broken signatures are likely
to be detected quickly using current versions of git.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>However, it also came with disadvantages:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Verifying a signed object requires access to the SHA-1 names of all
objects it references, even after the transition is complete and
translation table is no longer needed for anything else. To support
this, the design added fields such as "hash sha1 tree &lt;SHA-1 name&gt;"
and "hash sha1 parent &lt;SHA-1 name&gt;" to the SHA-256 content of a signed
commit, complicating the conversion process.</p>
</li>
<li>
<p>Allowing signed objects without a SHA-1 (for after the transition is
complete) complicated the design further, requiring a "nohash sha1"
field to suppress including "hash sha1" fields in the SHA-256 content
and signed payload.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_lazily_populated_translation_table">Lazily populated translation table</h3>
<div class="paragraph">
<p>Some of the work of building the translation table could be deferred to
push time, but that would significantly complicate and slow down pushes.
Calculating the SHA-1 name at object creation time at the same time it is
being streamed to disk and having its SHA-256 name calculated should be
an acceptable cost.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_document_history">Document History</h2>
<div class="sectionbody">
<div class="paragraph">
<p>2017-03-03
<a href="mailto:bmwill@google.com">bmwill@google.com</a>, <a href="mailto:jonathantanmy@google.com">jonathantanmy@google.com</a>, <a href="mailto:jrnieder@gmail.com">jrnieder@gmail.com</a>,
<a href="mailto:sbeller@google.com">sbeller@google.com</a></p>
</div>
<div class="ulist">
<ul>
<li>
<p>Initial version sent to <a href="https://lore.kernel.org/git/20170304011251.GA26789@aiede.mtv.corp.google.com" class="bare">https://lore.kernel.org/git/20170304011251.GA26789@aiede.mtv.corp.google.com</a></p>
</li>
</ul>
</div>
<div class="paragraph">
<p>2017-03-03 <a href="mailto:jrnieder@gmail.com">jrnieder@gmail.com</a>
Incorporated suggestions from jonathantanmy and sbeller:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Describe purpose of signed objects with each hash type</p>
</li>
<li>
<p>Redefine signed object verification using object content under the
first hash function</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>2017-03-06 <a href="mailto:jrnieder@gmail.com">jrnieder@gmail.com</a></p>
</div>
<div class="ulist">
<ul>
<li>
<p>Use SHA3-256 instead of SHA2 (thanks, Linus and brian m. carlson).[1][2]</p>
</li>
<li>
<p>Make SHA3-based signatures a separate field, avoiding the need for
"hash" and "nohash" fields (thanks to peff[3]).</p>
</li>
<li>
<p>Add a sorting phase to fetch (thanks to Junio for noticing the need
for this).</p>
</li>
<li>
<p>Omit blobs from the topological sort during fetch (thanks to peff).</p>
</li>
<li>
<p>Discuss alternates, git notes, and git servers in the caveats
section (thanks to Junio Hamano, brian m. carlson[4], and Shawn
Pearce).</p>
</li>
<li>
<p>Clarify language throughout (thanks to various commenters,
especially Junio).</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>2017-09-27 <a href="mailto:jrnieder@gmail.com">jrnieder@gmail.com</a>, <a href="mailto:sbeller@google.com">sbeller@google.com</a></p>
</div>
<div class="ulist">
<ul>
<li>
<p>Use placeholder NewHash instead of SHA3-256</p>
</li>
<li>
<p>Describe criteria for picking a hash function.</p>
</li>
<li>
<p>Include a transition plan (thanks especially to Brandon Williams
for fleshing these ideas out)</p>
</li>
<li>
<p>Define the translation table (thanks, Shawn Pearce[5], Jonathan
Tan, and Masaya Suzuki)</p>
</li>
<li>
<p>Avoid loose object overhead by packing more aggressively in
"git gc --auto"</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Later history:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>See the history of this file in git.git for the history of subsequent
edits. This document history is no longer being maintained as it
would now be superfluous to the commit log</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>References:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>[1] https://lore.kernel.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/
[2] https://lore.kernel.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/
[3] https://lore.kernel.org/git/20170306084353.nrns455dvkdsfgo5@sigill.intra.peff.net/
[4] https://lore.kernel.org/git/20170304224936.rqqtkdvfjgyezsht@genre.crustytoothpaste.net
[5] https://lore.kernel.org/git/CAJo=hJtoX9=AyLHHpUJS7fueV9ciZ_MNpnEPHUz8Whui6g9F0A@mail.gmail.com/</pre>
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